Cylinder-Type Linear Motor and Moving Part Thereof

ABSTRACT

The present invention provides a cylinder-type linear motor capable of shortening the total motor length with respect to a predetermined stroke length and capable of being operated as a brushless DC motor without a sensor means for sensing the position of a moving part being added in the axial direction. The present invention also provides a moving part of said cylinder-type linear motor, which can improve the magnetic flux density distribution waveform near both end portions of the moving part assembly and can improve the thrust characteristic by bringing the magnetic flux density distribution waveform closer to a cosine waveform and by increasing the amplitude of cosine waveform.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/197,014 filed on Aug. 4, 2005, and claims priority from JapanesePatent Application No. 2004-231993; filed Aug. 9, 2004; Japanese PatentApplication No. 2004-253765; filed Sep. 1, 2004; and Japanese PatentApplication No. 2004-253766; filed Sep. 1, 2004, the disclosures ofwhich are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

1. Field of the Invention

The present invention relates to a cylinder-type linear motor having apermanent magnet in a moving part section and a plurality of ring-shapedcoils in a fixed part section, and a moving part thereof.

2. Description of Related Art

FIG. 11 is a sectional view of a two-phase cylinder-type linear motorthat has been known conventionally.

In FIG. 11, a moving part 100 of the cylinder-type linear motor includesa linear motion shaft 101 reciprocating in the axial direction, acylindrical moving part yoke 102 mounted on the linear motion shaft 101,and a plurality of ring-shaped permanent magnets 103 arranged on theouter peripheral surface of the moving part yoke 102 in a magnetizedmanner such as to adopt an alternate polarity with a magnetic pole pitchP in the axial direction. Also, a fixed part core 201 of a fixed part200 has ring-shaped yoke portions 202 each having a smaller insidediameter and ring-shaped yoke portions 203 each having a larger insidediameter, both yoke portions being laminated alternately in the axialdirection. As a result, on the inner peripheral surface of the fixedpart 201, a large number of ring-shaped tooth portions 204 andring-shaped groove portions 205 are formed in the axial direction withan equal pitch (P/2). In the ring-shaped groove portions 205,ring-shaped coils 206, 207, . . . , 213 are disposed in the phase orderof A phase and B phase. Therefore, the ring-shaped coils 206, 208, 210and 212 disposed alternately are connected to each other to form anA-phase winding, and the remaining coils 207, 209, 211 and 213 are alsoconnected to each other to form a B-phase winding.

An axial length M of a moving part core section is longer than an axiallength K of a fixed core section, and therefore is an axial length inwhich the moving part and the fixed part face each other, namely, athrust contribution length K. Also, a stroke length S is expressed by(M−K). From FIG. 11, the travel range length of the moving part isexpressed by (K+2S), namely, (thrust contribution length+2×stroke lengthS). The total motor length is set so as to satisfy the travel rangelength of the moving part. As the related art, Japanese PatentProvisional Publication No. 7-107732 and Japanese Patent ProvisionalPublication No. 5-15139 can be cited.

OBJECT AND SUMMARY OF THE INVENTION

For the linear motor having the above-described conventionalconstruction, in order to provide a predetermined stroke S, it isnecessary to set the total length of the motor so as to satisfy thetravel range length (thrust contribution length+2×stroke length S) ofthe moving part, which results in a problem of greater total motorlength. Also, since the above-described motor having the conventionalconstruction is of a permanent magnet type, it can be operated as abrushless DC motor in principle. For this purpose, however, sensor meansfor sensing the position of moving part must be provided separately soas to be adjacent to the fixed part section in the axial direction. Inthis case, there arises a problem in that the total motor lengthincreases further.

In order to improve the workability in producing the fixed part, aconstruction is sometimes employed in which a comb teeth shaped corehaving a cross section equivalent to the cross section of the fixed partcore shown in FIG. 11 is fitted from the outside of the ring-shapedcoils. In this case, there arises a problem in that in order to changethe stroke length, it is necessary to prepare a mold newly, andtherefore the stroke length cannot be changed easily. Further, therealso arises a problem in that it is necessary to change the length offixed part so that the number of ring-shaped coils in windings of bothphases is equal, and therefore the degree of freedom in changing thestroke length is low. Still further, there arises a problem in that whenthe thrust contribution length is fixed, the inertia moment of movingpart increases, thereby degrading the response, as the stroke length Sis increased.

The present invention has been made to solve the above problems.Accordingly, it is an object of the present invention to provide acylinder-type linear motor in which the total motor length can beshortened with respect to a predetermined stroke length S, the linearmotor can be operated as a brushless DC motor without sensor means forsensing the position of a moving part being added in the axialdirection, the stroke length S can be increased/decreased easily at alow cost, the degree of freedom in changing the stroke length can bemade high, and the inertia moment of moving part is not affected by thestroke length S.

Also, it is another object of the present invention to provide a movingpart of a cylinder-type linear motor in which the total motor length canbe shortened with respect to a predetermined stroke length S, the linearmotor can be operated as a brushless DC motor without sensor means forsensing the position of a moving part being added in the axialdirection, and a magnetic pole position detecting method, which has beenused for the conventional brushless motor, can be adopted.

Further, it is still another object of the present invention to providea moving part of a cylinder-type linear motor in which the total motorlength can be shortened with respect to a predetermined stroke length S,the linear motor can be operated as a brushless DC motor without sensormeans for sensing the position of a moving part being added in the axialdirection, the magnetic flux density distribution waveform near both endportions of the moving part assembly can be improved, and the thrustcharacteristic can be improved by bringing the magnetic flux densitydistribution waveform closer to a cosine waveform and by increasing theamplitude of cosine waveform.

To solve the above problems, the present invention provides acylinder-type linear motor including a fixed part including a coilassembly having a plurality of (n number of) ring-shaped coils arrangedin the axial direction to form a cylindrical space, and a yoke membermade of a magnetic material, which is provided on the outer peripheryside of the coil assembly; and a moving part including a linear motionshaft provided on the axis line of the fixed part so as to be capable ofreciprocating in the axial direction, and a permanent magnet assemblyhaving one or more permanent magnets magnetized in the axial direction,which is provided on the linear motion shaft, characterized in that whenthe axial length of the ring-shaped coil is taken as C, the axial lengthof the permanent magnet assembly as M, and the outside diameter thereofas D, a stroke S is equal to or smaller than (n×C−M), the axial length Yof the yoke member is set equal to or larger than (M+S+0.8×D), and thering-shaped coils are arranged in a predetermined phase order and thering-shaped coils of the same phase are connected to each other to formone phase winding.

Also, in the cylinder-type linear motor in accordance with the presentinvention, the permanent magnet assembly of the moving part is arrangedso that when the number of permanent magnets is two or more, the endfaces with the same polarity face to each other.

Further, in the cylinder-type linear motor in accordance with thepresent invention, the yoke member is formed by a cylindrical member, anopening extending in the axial direction is provided in the cylindricalmember, and sensor means for sensing the magnetic pole position of themoving part is provided in the opening.

Still further, in the cylinder-type linear motor in accordance with thepresent invention, the yoke member is formed by a plurality of slenderplate-shaped members, these plate-shaped members are arranged atpredetermined intervals in the circumferential direction so as to coverthe outer peripheral surface of the coil assembly, and the coil assemblyis held by the plate-shaped members.

Further, in the cylinder-type linear motor in accordance with thepresent invention, a plurality of groove portions parallel with thelinear motion shaft are provided in the inner peripheral surface of analuminum-made case member for radiating heat from the coils, and theyoke members are disposed in the groove portions.

Still further, in the cylinder-type linear motor in accordance with thepresent invention, the linear motor is driven by a driving circuithaving sensor means for sensing the magnetic pole position of the movingpart, which is provided in the outer peripheral surface of the coilassembly; a moving part magnetic pole position detecting section fordetecting the magnetic pole position of the moving part by means of asignal from the sensor means; a memory section for storing pattern datathat are set so as to correspond to the moving part magnetic poleposition and to correct asymmetry of magnetic flux density distributionwaveform caused by the configuration of the permanent magnet assemblyand asymmetry of mating winding existing in the case where the number ofring-shaped coils is different according to the phase; a current controlsection in which a current command value of each phase is produced basedon the pattern data read from the memory section so as to correspond tothe moving part magnetic pole position and a current command from aspeed control section, the current command value is compared with anactual current value sent from a current detecting section for detectinga current flowing in each phase winding, and a gate signal for carryingout PWM control is generated so that the difference is zero; and aninverter section provided with switching means that is ON/OFF controlledby the gate signal from the current control section.

The present invention provides a moving part of a cylinder-type linearmotor, including a linear motion shaft which is arranged on an axis lineof a cylindrical fixed part so as to reciprocate on the axis line in theaxial direction; and a permanent magnet assembly which is provided onthe linear motion shaft and is configured so that a plurality ofpermanent magnets magnetized in the axial direction are disposed so thatthe end faces thereof face to each other, characterized in that a unitis formed by a first permanent magnet magnetized in a first axialdirection and second and third permanent magnets magnetized in thedirection opposite to the first axial direction, which are arranged onboth sides of the first permanent magnet; one or more units are arrangedin series in the axial direction to form the permanent magnet assembly;when the first to third permanent magnets are arranged in the order ofthe second, first and third permanent magnets from the left, a distancebetween the left-hand side end face of the second permanent magnet andthe right-hand side end face of the third permanent magnet is set at2×L, and a distance between a first central position between theright-hand side end face of the second permanent magnet and theleft-hand side end face of the first permanent magnet and a secondcentral position between the right-hand side end face of the firstpermanent magnet and the left-hand side end face of the third permanentmagnet is set at L.

Also, in the moving part of a cylinder-type linear motor in accordancewith the present invention, the axial length of the first permanentmagnet is L, and the axial lengths of the second and third permanentmagnets each are L/2.

Further, in the moving part of a cylinder-type linear motor inaccordance with the present invention, the cross-sectional areas of thesecond and third permanent magnets are equal to each other, and each aredifferent from the cross-sectional area of the first permanent magnet.

Still further, in the moving part of a cylinder-type linear motor inaccordance with the present invention, the energy products of the secondand third permanent magnets are equal to each other, and each aredifferent from the energy product of the first permanent magnet.

To solve the above problems, the present invention provides a movingpart of a cylinder-type linear motor, including a linear motion shaftwhich is arranged on an axis line of a cylindrical fixed part so as toreciprocate on the axis line in the axial direction; and a permanentmagnet assembly which is provided on the linear motion shaft and isconfigured so that a plurality of permanent magnets magnetized in theaxial direction are disposed so that the end faces thereof face to eachother, characterized in that the permanent magnet assembly has a pair ofpermanent magnets magnetized in the axial direction, which are arrangedso that the end faces with the same polarity face to each other; a pairof ring-shaped fixing members each having an outside diameter smallerthan the outside diameter of the paired permanent magnets, which arefixed on the linear motion shaft so as to be in contact with both sidesof the paired permanent magnets; and a pair of ring-shaped permanentmagnets magnetized in the radial direction so that the polarity on theouter peripheral surface is different from the polarity on the opposedend face of the paired permanent magnets, which are provided on theouter peripheral surfaces of the paired ring-shaped fixing members.

Also, in the moving part of a cylinder-type linear motor in accordancewith the present invention, the ring-shaped fixing members each are madeof a magnetic material.

Further, in the moving part of a cylinder-type linear motor inaccordance with the present invention, a distance between centralpositions in the axial direction of the ring-shaped permanent magnets isset so as to be two times of a magnetic pole pitch of the moving part.

According to the present invention, since the cylinder-type linear motoris configured as described above, the travel range length of a movingpart that determines a necessary total motor length can be made (thrustcontribution length+stroke length S). Also, since the fixed part has noslot, a mold is not needed, the increase/decrease in stroke length doesnot depend on the length of the moving part, and the number ofring-shaped coils can be increased or decreased in unit of one coil.Therefore, the stroke length can be increased or decreased easily at alow cost, and the degree of freedom in changing the number ofring-shaped coils can be made high. Also, since the sensor means forsensing the position of moving part can be disposed on the outside ofthe ring-shaped coil, the linear motor can be operated as a brushless DCmotor without increasing the axial length.

According to the present invention, the distribution waveform of radialcomponent of magnetic flux density is a synthesis of distributionwaveforms of permanent magnets constituting the permanent magnetassembly of the moving part. Therefore, by setting the distributionwaveform formed by the second and third permanent magnets with respectto the distribution waveform formed by the first permanent magnet, thesynthesized distribution waveform can be improved, so that a magneticpole position detecting method, which has been used for conventionalbrushless motors, can be adopted.

Also, according to the present invention, since the cylinder-type linearmotor is configured as described above, the travel range length of amoving part that determines a necessary total motor length can be made(thrust contribution length+stroke length S).

According to the present invention, the magnetic flux densitydistribution waveform near both end portions of the moving part assemblycan be improved, and the thrust characteristic can be improved bybringing the magnetic flux density distribution waveform closer to acosine waveform and by increasing the amplitude of cosine waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view showing an embodiment of acylinder-type linear motor in accordance with the present invention;

FIG. 2 is a longitudinal sectional view in which symbols designating thelengths of parts shown in FIG. 1 are shown;

FIG. 3 is a view schematically showing the relationship of a detentthrust depending on the relationship between a travel range length of amoving part and a length of a cylindrical yoke;

FIG. 4 is a view showing an axial distribution waveform of a radialcomponent of magnetic flux density due to a moving part in accordancewith the present invention and a permanent magnet assembly correspondingto the moving part;

FIG. 5 is a control block diagram for illustrating the drive of a linearmotor in accordance with the present invention;

FIG. 6 is a transverse sectional view showing a modification of acylinder-type linear motor in accordance with the present invention;

FIG. 7 is a longitudinal sectional view showing an embodiment of amoving part of a cylinder-type linear motor in accordance with thepresent invention;

FIG. 8 is a schematic view showing an embodiment of a moving part of acylinder-type linear motor in accordance with the present invention anda distribution curve;

FIG. 9 is a longitudinal sectional view showing another embodiment of amoving part of a cylinder-type linear motor in accordance with thepresent invention;

FIG. 10 is a schematic view showing another embodiment of a moving partof a cylinder-type linear motor in accordance with the present inventionand a distribution curve; and

FIG. 11 is a longitudinal sectional view showing a conventionalcylinder-type linear motor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedexemplarily and in detail with reference to the accompanying drawings.

FIGS. 1 and 2 are sectional views showing one embodiment of a linearmotor in accordance with the present invention.

In FIGS. 1 and 2, a moving part 10 includes a linear motion shaft 11reciprocating in the axial direction, and a permanent magnet assembly 15in which two permanent magnets 12 and 14 magnetized in the axialdirection are disposed with a spacer 13 being held therebetween so thatthe end surfaces thereof with the same polarity face to each other. Thespacer 13 adjusts a magnetic pole pitch P (distance between the N and Spoles) of the moving part 10, and also regulates a waveform (magneticflux density distribution waveform) in which a radial component ofmagnetic flux density changes according to the axial position of movingpart. The spacer 13 may be made of a magnetic material or may be made ofa non-magnetic material. In some cases, the spacer 13 need not be used.

The permanent magnet assembly 15 is firmly fixed to the linear motionshaft 11 by fixing ring members 16 and 17 provided on both end faces ofthe permanent magnet assembly 15. The moving part 10 is supported in thecenter of a pair of brackets 30 and 31 facing to each other with apredetermined distance via bearings 32 and 33 so as to be movable in theaxial direction. On the other hand, a fixed part 20 both ends of whichare supported by the brackets 30 and 31 is composed of a cylindricalyoke 21 made of a magnetic material, and a coil assembly 28 having aplurality of ring-shaped coils 22, 23, . . . , 27 disposed in the axialdirection with an equal pitch C. The coil assembly 28 is housed in thecylindrical yoke 21, and is formed with a cylindrical space 34 forcontaining the permanent magnet assembly 15 therein. Also, thecylindrical yoke 21 is provided with an opening 29 extending in theaxial direction, and a circuit board 40 having sensor means 41, notshown, for sensing the magnetic pole position of the moving part 10 isinstalled therein. The circuit board 40 is provided with a terminal forconnecting the starting end and trailing end of each of the coils, andthe coils are connected by a printed circuit provided on the circuitboard 40 so that the winding of each phase is formed.

FIG. 3 is a view schematically showing the relationship of a detentthrust depending on the relationship between a travel range length (M+S)of the moving part 10 and a length Y of the cylindrical yoke 21. In FIG.3, the center in the axial direction of the permanent magnet assembly 15is taken as an origin of position. From FIG. 3, it can be seen that adetent thrust that draws the moving part 10 into the yoke is generatednear both ends of the stroke. Also, it can be seen that if the length Yof the cylindrical yoke 21 is set so as to be equal to or greater than acertain value Yo, the detent thrust can be kept at a negligible value inthe travel range length (M+S) of the moving part 10. A yoke projectingdimension B at the time when Y is equal to Yo depends on the outsidediameter D of the permanent magnet, and is expressed as B=kD. It isdetermined analytically that k is proper when it takes a value of about0.4. Therefore, Yo=M+S+2×B=M+S+0.8×D.

From the above description, as shown in FIG. 2, when the axial length ofthe permanent magnet assembly 15 is taken as M, the outside diameterthereof as D, and the stroke length as S, the length Y of thecylindrical yoke 21 is set at a value equal to or larger than(M+S+0.8×D). By doing this, the detent thrust generated at both ends ofstroke at the time of de-energization can be kept at a negligible value.When the pitch between the ring-shaped coils 22, 23, . . . , 27 is takenas C, since the number n of the ring-shaped coils 22, 23, . . . , 27 issix, an axial length K of the fixed part 20 is 6C. In this case, thethrust contribution length is M. Also, the stroke length S can take avalue equal to or smaller than (K−M), namely, (6C−M). In FIG. 2, thenumber of the ring-shaped coils 22, 23, . . . , 27 is six. However, byincreasing the number of the ring-shaped coils 22, 23, . . . 27 to sevenor decreasing to five, for example, the stroke length S can be increasedor decreased with the coil pitch C being a unit to (7C−M) or (5C−M).Since the stroke length S does not depend on the length M of thepermanent magnet assembly 15, it can be seen that even if the strokelength S is increased, the inertia moment of the moving part 10 does notincrease. Also, since the travel range length of the moving part 10 is(thrust contribution length+stroke length S), the total motor length canbe shortened by the stroke length S as compared with the conventionalexample.

FIG. 4 is a view showing an axial distribution waveform of a radialcomponent of magnetic flux density due to the moving part 10 inaccordance with this embodiment and the permanent magnet assembly 15corresponding to the moving part 10. It can be seen that in the range ofthe axial length M of the permanent magnet assembly 15, the distributionwaveform of magnetic flux density can be approximated as a waveform inwhich a DC component is added to a cosine component. Specifically, whenthe axial position with the axial center position of the permanentmagnet assembly 15 being the origin is taken as (x), the magnetic fluxdensity B(x) is expressed as B(x)=B1×cos(πx/L)+B0, wherein L is amagnetic pole pitch of moving part. The output voltage of the sensormeans 41 is also a voltage proportional to this, so that the position ofthe moving part 10 can be read by utilizing the same relationalexpression as described above.

Also, it can be seen that since the thrust is proportional to theproduct of counter electromotive voltage and current, and the counterelectromotive voltage is proportional to a change with respect to theposition of the magnetic flux density, an influence of the DC componentof the magnetic flux density is eliminated.

FIG. 5 is a control block diagram for illustrating the drive of thelinear motor in accordance with the present invention.

A signal 42 generated from the sensor means 41 incorporated in a motorsection 60 is sent to a moving part magnetic pole position detectingsection 43 and a moving part speed detecting section 44, where themagnetic pole position and speed of the moving part 10 are detected.Detected moving part speed data 45 and command speed data 46 are inputinto a speed control section 47, and a current command 48 correspondingto a difference between a command speed and an actual moving part speedis sent out of the speed control section 47. On the other hand, magneticpole data is sent out of the moving part magnetic pole positiondetecting section 43, and is input into a memory section 49 that storesthree-phase pattern data set corresponding to the moving part magneticpole position. Data 50 corresponding to the moving part position is sentout of the memory section 49, and is input into a current controlsection 51 together with the current command 48, whereby a command valueof each phase current is generated. In the current control section 51,the generated current command is compared with actual each phase currentvalue 52 detected by a current detecting section 56, and a gate signal53 for carrying out PWM control is generated so that the difference iszero. The gate signal 53 controls ON/OFF of switching means of aninverter section 54, whereby control is carried out so that each phasecurrent waveform of the motor section 60 takes a predetermined value.The memory section 49 stores the aforementioned three-phase pattern dataconsidering the correction of asymmetry of magnetic flux densitydistribution waveform caused by the configuration of the permanentmagnet assembly 15 and asymmetry of phase winding caused by theoccurrence of a difference in the number of coils constituting the phasewinding to obtain a predetermined stroke length. Therefore, theincrease/decrease in stroke can be effected in unit of one coil, andalso smooth speed control can be achieved. Also, it is a matter ofcourse that positioning control can also be carried out by adding aposition control loop to the control block diagram shown in FIG. 5.

The aforementioned yoke member need not necessarily be of a cylindricalshape. As shown in a transverse sectional view of FIG. 6, slenderplate-shaped yoke members 70, 71, 72 and 73 may be brought into contactwith the outer peripheral surface of the coil assembly 28 at intervalsin the circumferential direction. The yoke members 70, 71, 72 and 73 areinstalled so as to be inserted in spaces 91, 92, 93 and 94 formedbetween groove portions 81, 82, 83 and 84 parallel with the linearmotion shaft 11, which are provided in the inner peripheral surface ofan aluminum case 80, and the coil assembly 28, respectively. Thealuminum case 80 is provided with an opening 85 that is long in theaxial direction, and the circuit board 40 mounted with the sensor means41 is disposed in the opening 85. In this case, by adjusting the ratioof the contact area with the aluminum case 80 to the contact area withthe yoke members 70, 71, 72 and 73, the thrust can be increased whilethe heat radiation property is improved.

In the cylinder-type linear motor having the above-described moving partconstruction, for example, when the sensor means is a Hall element,since the output waveform of sensor means is proportional to the radialcomponent of the magnetic flux density at a space position of the sensormeans, the waveform becomes as indicated by a curve V in FIG. 4.Therefore, there arises a problem in that a system such that a zerocross point of the output waveform is detected to detect the magneticpole position, which system having been used in the conventionalbrushless motor, cannot be used. For this reason, it is necessary todetect the moving part position by using storing means that stores therelationship between the output waveform and the moving part position inadvance, which also presents the problem of a complicated circuit.

FIG. 7 shows a cylinder-type linear motor in which the construction ofmoving part is changed. In FIG. 7, the same reference numerals areapplied to elements that are the same as those in FIGS. 1 to 4, andexplanation of those elements is omitted.

FIG. 8 is a partial sectional view of FIG. 7, showing one embodiment ofthe moving part construction of the cylinder-type linear motor inaccordance with the present invention, and a diagram showing magneticflux density distribution waveforms of permanent magnets correspondingto the moving part construction and a composite waveform thereof. Inthis case, the central position in the axial direction of a firstpermanent magnet is the origin on the horizontal axis.

In FIGS. 7 and 8, a moving part 1 includes the linear motion shaft 11reciprocating in the axial direction; a permanent magnet assembly 5consisting of a first permanent magnet 3 magnetized in the first axialdirection and second and third permanent magnets 2 and 4 magnetized inthe direction opposite to the first axial direction, which are arrangedon both sides of the first permanent magnet 3; and the fixing ringmembers 16 and 17 provided on both sides of the permanent magnetassembly 5.

On the other hand, the fixed part 20 both ends of which are supported bythe brackets 30 and 31 is composed of the cylindrical yoke 21 made of amagnetic material, and the coil assembly 28 having the plurality ofring-shaped coils 22, 23, . . . , 27 disposed in the axial directionwith the equal pitch C. The coil assembly 28 is housed in thecylindrical yoke 21, and is formed with the cylindrical space 34 forcontaining the moving part 1 provided with the permanent magnet assembly5 therein. The moving part 1 is arranged on the axis line of thecylindrical space 34, and is supported so as to reciprocate in the axialdirection.

Also, the cylindrical yoke 21 is provided with the opening 29 extendingin the axial direction, and the circuit board 40 having sensor means,not shown, for sensing the magnetic pole position of the moving part 1is installed therein.

When the axial length of the first permanent magnet 3 is taken as L, thedistribution waveform of magnetic flux density formed by the firstpermanent magnet 3 is represented by a curve W shown in FIG. 8. Thepeaks of magnetic flux density occur at the axial positions of ±L/2, andthe magnetic pole pitch (distance between the N and S poles) P is equalto L. At this time, the magnetic flux density at the axial positions of±L is not zero, but has a value of ±K as shown in FIG. 8. If the axiallengths of the second and third permanent magnets 2 and 4 are set at L/2at this time, the magnetic flux distribution formed by the secondpermanent magnet 2 is represented by a curve X, the peaks thereofoccurring at the axial positions of (−L) and (−L/2).

Also, the magnetic flux distribution formed by the third permanentmagnet 4 is represented by a curve Y, the peaks thereof occurring at theaxial positions of (L) and (L/2). If the outside diameters of the secondand third permanent magnets 2 and 4 are made smaller than the outsidediameter of the first permanent magnet 3, the cross-sectional areas ofthe second and third permanent magnets 2 and 4 become smaller than thecross-sectional area of the first permanent magnet 3. Further, thedistance between the second and third permanent magnets 2 and 4 and thesensor means becomes longer than in the case of the first permanentmagnet 3. By these two facts, the peak values of the second and thirdpermanent magnets 2 and 4 can be made smaller than the peak values ofthe first permanent magnet 3, so that the outside diameters thereof canbe adjusted so that the peak values are substantially close to thevalues of ±K.

The magnetic flux density distribution waveform synthesized by thesethree permanent magnets 2, 3 and 4 is represented by a curve Z shown inFIG. 8, namely, a waveform close to a sinusoidal wave distribution canbe obtained. As a result, a magnetic pole position detecting system,which has been used in the conventional brushless motor, can be used.

The method for adjusting the peak values of magnetic flux densitydistribution of the second and third permanent magnets 2 and 4 is notmerely to decrease the outside diameters of magnets as in theabove-described embodiment, but may be to inversely increase the outsidediameters depending to the space position of the sensor means. Also, asthe method for changing the cross-sectional area, the inside diametersof the magnets may be changed. Alternatively, without changing thecross-sectional area, the materials of the second and third permanentmagnets 2 and 4 may be changed to decrease or increase the energyproducts thereof as compared with the energy product of the firstpermanent magnet 3. Further, these three methods may be used combinedly.

Although an example in which the unit number of permanent magnetassemblies is one has been described in this embodiment, it is a matterof course that the unit number may be plural.

However, in the cylinder-type linear motor having the above-describedmoving part construction, as shown in FIG. 4, the magnetic flux densitydistribution near both end portions of the moving part assembly greatlydeviates from a cosine waveform, so that the thrust characteristic issometimes deteriorated. Also, there arises a problem in that the rangein which the aforementioned approximate expression ofB(x)=B1×cos(πx/L)+BO can be applied is limited to the range of 2×L shownin the figure.

FIG. 9 shows a moving part of the cylinder-type linear motor inaccordance with the present invention, which can improve the magneticflux density distribution waveform near both end portions of the movingpart assembly by changing the construction of the moving part, and canimprove the thrust characteristic by bringing the waveform closer to acosine waveform and by increasing the amplitude of the cosine waveform.In FIG. 9, the same reference numerals are applied to elements that arethe same as those in FIGS. 1 to 4, and explanation of those elements isomitted.

FIG. 10 is a partial sectional view of FIG. 9, showing one embodiment ofthe moving part of the cylinder-type linear motor in accordance with thepresent invention, and a diagram showing magnetic flux densitydistribution waveforms of permanent magnets corresponding to the movingpart construction and a composite waveform thereof. In this case, thecentral position in the axial direction of the second and thirdpermanent magnet is the origin on the horizontal axis.

In FIGS. 9 and 10, a moving part 1 includes the linear motion shaft 11reciprocating in the axial direction; a pair of permanent magnets 2 and4 consisting of the permanent magnet 2 magnetized in the axial direction(left to right) and the permanent magnet 4 magnetized in the oppositedirection (right to left), which is arranged so as to face to thepermanent magnet 2; the paired fixing ring members 6 and 7 which arebrought into contact with both end faces of the paired permanent magnets2 and 4 and are fixed on the linear motion shaft 11; a pair ofring-shaped permanent magnets 8 and 9 which are provided so as to befitted on or brought into contact with the outer peripheral surfaces ofthe fixing ring members 6 and 7. The paired ring-shaped permanentmagnets 8 and 9 are magnetized in the radial direction so that thepolarity of outer peripheral surface is different from the polarity ofthe opposed surfaces of the paired permanent magnets 2 and 4. Also, thedistance between the central positions of the axial length of thering-shaped permanent magnets 8 and 9 is set at 2×L, and the outsidediameters thereof are set so as to be equal to the outside diameters ofthe paired permanent magnets 2 and 4.

On the other hand, the fixed part 20, both ends of which are supportedby the brackets 30 and 31, is composed of the cylindrical yoke 21 madeof a magnetic material, and the coil assembly 28 having the plurality ofring-shaped coils 22, 23, . . . , 27 disposed in the axial directionwith the equal pitch C. The coil assembly 28 is housed in thecylindrical yoke 21, and is formed with the cylindrical space 34 forcontaining the moving part 1 provided with the permanent magnet assembly5 therein. The moving part 1 is arranged on the axis line of thecylindrical space 34, and is supported so as to reciprocate in the axialdirection.

Also, the cylindrical yoke 21 is provided with the opening 29 extendingin the axial direction, and the circuit board 40 having sensor means,not shown, for sensing the magnetic pole position of the moving part 1is installed therein.

Since the moving part 1 is configured as described above, thedistribution waveform of magnetic flux density formed by the pairedpermanent magnets 2 and 4 is represented by a curve W in FIG. 10, andthe magnetic flux density distribution waveform formed by the pairedring-shaped permanent magnets 8 and 9 is represented by a curve X. Ifthe values of magnetic density at point A of the curve W and at point Bof the curve X are set so as to cancel each other, the synthesizedmagnetic flux density distribution waveform can be made a curve Y, andthe magnetic flux density distribution waveform near both end portionsof the permanent magnet assembly 5 can be brought closer to a cosinewaveform.

The ring-shaped permanent magnets 8 and 9 are not limited to thoseformed into a cylindrical shape. Arc-shaped permanent magnets may beaffixed to each other to form a ring shape. Also, an appropriate spacermay be interposed between the ring-shaped permanent magnet 8 and thepermanent magnet 2, between the permanent magnet 2 and the permanentmagnet 4, and between the permanent magnet 4 and the ring-shapedpermanent magnet 9.

As the result of the above configuration, the construction shown in FIG.9 can be achieved without changing the length of the moving part 1including the fixing ring members 6 and 7 shown in FIG. 10, so that thecharacteristic can be improved without changing the length of the movingpart.

The present invention is not limited to the above-described embodiments,and it is a matter of course that changes and modifications can be madeappropriately without departing from the spirit and scope of the presentinvention.

1. A moving part of a cylinder-type linear motor, comprising a linearmotion shaft which is arranged on an axis line of a cylindrical fixedpart so as to reciprocate on the axis line in the axial direction; and apermanent magnet assembly which is provided on the linear motion shaftand is configured so that a plurality of permanent magnets magnetized inthe axial direction are disposed so that the end faces thereof face eachother, characterized in that a unit is formed by a first permanentmagnet magnetized in a first axial direction and second and thirdpermanent magnets magnetized in the direction opposite to the firstaxial direction, which are arranged on both sides of the first permanentmagnet; one or more units are arranged in series in the axial directionto form the permanent magnet assembly; when the first to third permanentmagnets are arranged in the order of the second, first and thirdpermanent magnets from the left, a distance between the left-hand sideend face of the second permanent magnet and the right-hand side end faceof the third permanent magnet is set at 2×L, and a distance between afirst central position between the right-hand side end face of thesecond permanent magnet and the left-hand side end face of the firstpermanent magnet and a second central position between the right-handside end face of the first permanent magnet and the left-hand side endface of the third permanent magnet is set at L.
 2. The moving part of acylinder-type linear motor of claim 1, characterized in that the axiallength of the first permanent magnet is L, and the axial lengths of thesecond and third permanent magnets each are L/2.
 3. The moving part of acylinder-type linear motor of claim 1, characterized in that thecross-sectional areas of the second and third permanent magnets areequal to each other, and each are different from the cross-sectionalarea of the first permanent magnet.
 4. The moving part of acylinder-type linear motor of claim 1, characterized in that the energyproducts of the second and third permanent magnets are equal to eachother, and each are different from the energy product of the firstpermanent magnet.